Genetics:

Mendel & The Gene Idea

Mendelian genetics

Character (heritable feature, i.e., fur color)

Trait (variant for a character, i.e., brown)

True-breed (all offspring of same variety)

Hybridization – (crossing of 2 different true-breds)

P generation (parents) F1 generation (first filial generation) F2- the next generation, etc….

Leading to the Law of Segregation

Alternative versions of genes (alleles) account for variations in inherited characteristics

For each character, an organism inherits 2 alleles, one from each parent

If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance

The alleles for each character segregate (separate) during gamete production (meiosis).

Genetic vocabulary……. Homozygous: pair of identical

alleles for a character.– Example: PP or pp– They are “true-breeding”

Heterozygous: two different alleles for a gene– Example- Pp

Phenotype: an organism’s traits– It describes what you “see”

Genotype: an organism’s genetic makeup (PP, Pp, or pp)

Testcross: breeding of a recessive homozygote X dominant phenotype (but unknown genotype)

The Law of Independent Assortment

Law of Segregation involves 1 character. What about 2 (or more) characters?

Monohybrid cross vs. dihybrid cross

The two pairs of alleles segregate independently of each other.

Mendel’s Law of Independent Assortment

Now you can do more predicting… Dihybrid Cross- A mating between 2 parents that are both

heterozygous for the same traits.

Ex. RrYy x RrYy– Always should give a 9:3:3:1 phenotypic ratio

In fact if we know the genotypes, we can use laws of probability:

Rule Of Multiplication- probability that independent events will occur simultaneously is a product of their individual probabilities

Rule Of Addition- probability of an event occurring in 2 or more independent ways is the sum of their individual probabilities.

Of course, Genetics is not purely Mendelian...

Incomplete dominance: results in a phenotype which is intermediate between the dominant and recessive trait. – Ex: pink snapdragons

Codominance: two alleles are equally expressed and affect the phenotype in separate, distinguishable ways. – Ex: Variegated flowers

but sometimes, even “Complete Dominance” really isn’t:

It’s like a continuum: example AA, Aa, aa

Complete Dominance

Co-dominanceIncomplete Dominance

AA andAa have same phenotype

“A” phenotypeand “a” phenotypeexpressed equally

Aa shows intermediatephenotype

Example: Tay-Sacs Disease Tay-sacs is a disease in which brain cells lack a

lipid metabolizing enzyme. It all depends on the level at which the phenotype

is studied: Organismal level- “complete dominance”

– Heterozygote is symptom-free because half the number of functional enzymes is enough to do the job.

Biochemical Level- “Incomplete Dominance” – Lab Enzyme reaction rates are intermediate for carriers.

Molecular Level- “Codominance” – Both functional and dysfunctional versions of the

enzyme are produced.

More non-mendalian…. Pleiotropy: ability of a single gene to

have multiple phenotypic effects. – Ex: sickle-cell anemia & malaria

resistance Epistasis: a gene at one locus affects

the phenotypic expression of a gene at a second locus. – Dihybrid cross deviates from 9:3:3:1– Ex: mice coat color

Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character – Ex: human skin pigment and height

Multiple alleles: more than 2 possible alleles for a gene. – Ex: human blood types– A, B, AB, O(A&B codominant, both dominant to O)

Human disorders

The family pedigree Recessive disorders:

•Cystic fibrosis•Tay-Sachs•Sickle-cell

Dominant disorders:•Huntington’s

Testing:•amniocentesis•chorionic villus sampling (CVS)